Grep time command output

Question

Using time ls, I have the following output:

$ time ls -l 
total 2
-rwx------+ 1 FRIENDS None 97 Jun 23 08:59 location.txt
-rw-r--r--+ 1 FRIENDS None 10 Jun 23 09:06 welcome
real    0m0.040s
user    0m0.000s    
sys     0m0.031s

Now, when I try to grep only the real value line, the actual result is:

$ time ls -l | grep real
real    0m0.040s
user    0m0.000s
sys     0m0.031s

My question is, how to get only the real value as output? In this case, 0m0.040s.

Answer 1

time writes its output to stderr, so you need to pipe stderr instead of stdout. But it's also important to remember that time is part of the syntax of bash, and it times an entire pipeline. Consequently, you need to wrap the pipeline in braces, or run it in a subshell:

 $ { time ls -l >/dev/null; } 2>&1 | grep real
 real   0m0.005s

With Bash v4.0 (probably universal on Linux distributions but still not standard on Mac OS X), you can use |& to pipe both stdout and stderr:

{ time ls -l >/dev/null; } |& grep real

Alternatively, you can use the time utility, which allows control of the output format. On my system, that utility is found in /usr/bin/time:

/usr/bin/time -f%e ls -l >/dev/null 

man time for more details on the time utility.

Answer 2

(time ls -l)  2>&1 > /dev/null |grep real

This redirects stderr (which is where time sends its output) to the same stream as stdout, then redirects stdout to dev/null so the output of ls is not captured, then pipes what is now the output of time into the stdin of grep.

Answer 3

If you just want to specify the output format of time builtin, you can modify the value of TIMEFORMAT environment variable instead of filtering it with grep.

In you case,

TIMEFORMAT=%R
time ls -l

would give you the "real" time only.

Here's the link to relevant information in Bash manual (under "TIMEFORMAT").

This is a similar question on SO about parsing the output of time.

Answer 4

Look out.. bash has a built-in "time" command. Here are some of the differences..

# GNU time command (can also use $TIMEFORMAT variable instead of -f)
bash> /usr/bin/time -f%e ls >/dev/null
0.00


# BASH built-in time command (can also use $TIME variable instead of -f)
bash> time -f%e ls >/dev/null
-f%e: command not found

real    0m0.005s
user    0m0.004s
sys     0m0.000s